This invention relates to a piston and piston ring arrangement for a two-cycle internal combustion engine and more particularly to a method for forming the piston and specifically its ring and piston pin boss area.
As is well known, the pistons of internal combustion engines are subject to considerable thermal stress which can cause a number of problems. Although some of these problems are experienced in all types of engines, certain problems arc also encountered that are more problematical in two-cycle engines.
One type of problem which can be experienced in all engines but in some instances may be more likely in two-cycle engines is a condition called "piston ring sticking." As is well known, the seal between the piston and the cylinder is normally accomplished by one or more piston rings that arc received in grooves formed in the head of the piston. The head of the piston is, of course, the most highly heated part, and thus heat transfer to the piston rings can present problems.
The piston rings are normally split so that they can be installed into the ring groove through expansion. The ring is then compressed when the piston and assembled rings are placed into the cylinder bore. In normal engine running, there is a small gap between the split ends of the piston rings. The facing ends of the piston ring adjacent the gap will become heated during normal engine operation to an extent that is greater than the remainder of the ring. Thus, lubricant which may be present between the gaps can carbonize or solidify. The buildup of such lubricant in the gap can eventually close the gap and cause the piston ring sticking condition.
It is, therefore, a principal object of this invention to provide an improved method of forming, a piston and ring construction for an internal combustion engine wherein piston ring sticking can be avoided or minimized.
The piston is normally connected to the small end of a connecting rod by means of a piston pin. The piston pin extends through piston pin bosses formed in the piston. These bosses are normally positioned below the head of the piston and at least in substantial part inwardly from the skirt of the piston. Therefore, by their very location, the piston pin bosses can become highly heated, and this heat is transmitted through the piston pin to the small end of the connecting rod. This gives rise to obvious heat problems and the possibility of deterioration of the lubricant in the small end of the connecting rod and potential damage.
It is, therefore, a still further object of this invention to provide an improved method of forming a piston for an internal combustion engine wherein the heating of the piston pin bosses is substantially reduced.
It is a further object of this invention to provide an improved method of forming a piston for an internal combustion engine wherein heat transfer to the piston pin bosses is reduced.
As has been noted, it is the normal practice to form a piston for an internal combustion engine with a plurality of ring grooves in which piston rings are received. Normally the uppermost groove or grooves receive piston rings that serve as compression seals. Lower grooves may receive oil scraper rings for controlling the amount of lubricant on the cylinder wall surfaces and for spreading the lubricant over the piston and piston rings for their lubrication.
The nature of the piston rings and the cooperation with the cylinder bore tends to cause the area below the top compression ring to receive little lubricant. Because of this, the compression ring may not be cooled adequately, nor may this area be adequately lubricated.
It is, therefore, a still further object of this invention to provide an improved method of forming a piston wherein the area around the uppermost piston rings is treated so as to entrap and hold lubricant.